The present invention relates to a vacuum-type circuit breaker, and more particularly, to a construction of the contacts of a circuit breaker of that type.
A phenomenon may occur in a vacuum-type circuit breaker such that an arc for circuit breaking becomes unstable and the electric current suddenly disappears before the natural current zero during repeated interruptions of the electric current. This is the chopping phenomenon, and if such the chopping current is high, then a surge voltage is caused at the load side, thereby presenting a great threat of electrical breakdown of the load. Chopping phenomenon never occurs when the current is satisfactorily flowing between the contacts, but if the current flow is reduced, the chopping phenomenon occurs. In other words, while current flow is satisfactory between the contacts, metallic vaporization takes place vigorously with a result that the arc is maintained stable. However, if the current flow is reduced, the volume of metallic vaporization is decreased, and the arc becomes unstable, thus resulting in an occurrence of the chopping phenomenon. A vacuum-type circuit breaker has a high arc-extinguishing capacity in vacuum, and hence, the chopping phenomenon is usually unavoidable. Consequently, it is an important factor on how to reduce chopping current. In order to do this, it is required to improve the properties of material forming the contacts. A contact should be provided with excellent electric and thermal conductivities. Hence, it is necessary to add a material having a high vapor pressure to said material of excellent electric and thermal conductivities in order to reduce the chopping current. Furthermore, the material to be added may preferably have a lower melting point than the principal component material of excellent electric and thermal conductivities and, in addition, have little or no solubility in solid principal component material. A contact composed of an alloy provided with the required properties as described above has a structure of the principal component material dispersed with the added material, and maintains the arc in a stable condition by vaporization of the added material at the time of interruption of the current. The typical alloy is Cu-Bi alloy or Cu-Pb alloy. However, in the use of said alloys, the chopping current has not been so low that electric breakdown of the load can be fully prevented.
In view of the above, we have been engaged in a development of a novel alloy to replace the Cu-Bi alloy or Cu-Pb alloy, and have discovered that when an alloy of the principal component material of copper to which a Bi-Pb alloy is added is used, the chopping current is low at the initial stage of interruption of the current. However, it has been found that in the use of this newly developed alloy, if the number of times of interruption becomes large, the chopping current goes up extremely high.